Feedback control is a control method for stably converging the state of a controlled object to a predetermined target state by that a feedback loop is formed with a controller and the controlled object. In feedback control, however, the controller unilaterally controls the controlled object to forcibly transform it into a target state. It is, therefore, a prerequisite that the dynamic behavior (dynamics) of the controller and controlled object can be clearly separated. It has hence been effectively used only when the prerequisite can be fulfilled.
With increase in size and complexity of artificial systems in recent years, there are increasing number of cases where a controller and a controlled object cannot be necessarily separated clearly. The tendency is strong in systems including human beings as part of it. The dynamics of the controller and controlled object mutually interfere with each other. Such situations are referred to as nonlinearity of system, and there has been used a method in which a controller and a controlled object are separated with a linear approximation and used in conventional feedback control. Also, a linear approximation and a feedback control have been unreasonably applied to a nonlinear system which does not fulfill the prerequisite or a system having so strong nonlinearity that a linear approximation cannot be substantially used to it.
In such circumstances, there has been proposed a system which does not unreasonably linearize a nonlinear system but positively utilize its nonlinearity. In a nonlinear system, when dynamics of a controller and a controlled object mutually interfere with each other, nonlinear vibrations (limit cycles, for example) in which the states thereof vibrationally fluctuate with time and an entrainment effect created through an interaction therebetween are often observed. There is an attempt to utilize such nonlinear vibrations and a nonlinear interaction therebetween. For example, an example in which nonlinearity is used for control of the walking motion of a bipedal robot to stabilize the walking motion (see Non-patent Document 1) and an example in which such nonlinearity is applied to assistance for communication between human beings (Patent Document 1) have been reported. The effectiveness of such systems utilizing nonlinear vibrations or a synchronous effect created through an interaction therebetween (which is referred to as “entrainment effect”) for improvement in the global stability of the walking motion against changes in environment and for improvement in realism in communication between human beings has been reported. This is the effect achieved by avoiding applying linear approximation unreasonably to achieve feedback control thereof.
[Patent Document 1] JP-A-2000-349920 (sections 0004 and 0018 to 0025, FIG. 1 to FIG. 6, etc.)
[Non-patent Document 1] “Search for Design Principle of Biological System,” Gentaro Taga, Mathematical Science No. 394, pp. 5 to 13, published in 1996 by SAIENSU-SHA. Co. Ltd.